Liquid electrolytes tend to suffer from problems relating to the loss of electrolyte from the cell case and require the use of cell seals. Under extreme conditions, for example during short circuit abuse of cells, the local heating within the cell case can lead to seal rupture, even with the most sophisticated seal and cell case design.
In general, solid electrolytes offer many advantages over liquid electrolytes such as more efficient use of space (no separator required), no risk of electrolyte spillage, simplification of seal design with resulting reduction of cost, extended temperature range of cell operation, better cathode operation and improved abuse resistance. Typical disadvantages of some solid electrolytes are that they tend to have a relatively low ionic conductivity and a poor mechanical compatibility with certain types of electrode materials.
Such electrolytes may be used in coulometers, smart windows or displays, cells and batteries.
During the last two decades a remarkable international research effort has been dedicated to studies in the domain of solvent-free solid polymer electrolytes based on lithium salts. For example, such research is discussed in Polymer Electrolyte Reviews—1 and 2, J. R. MacCallum and C. A. Vincent (Elsevier, New York, 1987 and 1989), Solid Polymer Electrolytes, Fundamentals and Technological Applications, F. M. Gray (VCH, New York, 1991), Applications of Electroactive Polymers, B. Scrosati (Chapman and Hall, London, 1993), Electrochemical Science and Technology of Polymers—1 and 2, R. G. Linford (Elsevier, London, 1987 and 1991) and Polymer Electrolytes, RSC Monographs, F. M. Gray (Royal Society of Chemistry, London, 1997).
The motivation for this research has been provided largely by the objective of developing novel lithium-based primary and secondary cells.
Advantages of polymer electrolytes typically include low processing costs, excellent mechanical properties, the possibility of using a large area/thin film format, variable cell configuration, low device weight, good chemical stability, good thermal stability and access to lithium chemistry.
However, whilst polymer electrolytes offer several important advantages over conventional liquid non-aqueous systems, the first generation of lithium ion conducting solvent-free polymer electrolytes also suffered from severe drawbacks.
Poly(ethylene oxide) (PEO) was the first polymer in which salt solubilisation and ionic transport were recognised.
D. E. Fenton, J. M. Parker and P. V. Wright reported the preparation of crystalline complexes of sodium and potassium salts with poly(ethylene oxide) in Polymer 14 (1973) 589.
P. V. Wright investigated the electrical conductivities of various ionic complexes of poly(ethylene oxide) in Br. Polymer J. 7 (1975) 319-327.
Poly(ethylene oxide) is still the most thoroughly characterised electrolyte host matrix today. However, in spite of the promise of initial results, systems based on poly(ethylene oxide) were found to crystallise as salt-polymer complexes or free polymer, resulting in a less favourable conductivity behaviour in certain ranges of salt concentrations and over a critical range of operating temperatures.
During the years which followed the introduction of the solid polymer electrolyte (essentially solvent-free systems) a remarkable number of different polymer hosts were studied, and, surprisingly it was soon widely accepted that the most effective host polymer structure was in fact the CH2CH2O group. Whilst the capacity of this group to co-ordinate guest species is exceptional, high molecular weight samples crystallise with a significant reduction in the observed conductivity.
It has been found that ion conductivity is confined to the amorphous domains of a polymer electrolyte and that the tendency of the polymer electrolyte to crystallise results in difficulties in the reproducible preparation of electrolytes (G. G. Cameron, M. D. Ingram and K. Sarmouk, Eur. Polym. J. 26 (1990) 1097-1101).
Various strategies have been applied to attenuate the disadvantages of electrolytes based on the CH2CH2O structural unit and electrolytes containing many different salts, polymer architectures and plasticising additive formulations have been reported and reviewed, for example in Polymer Electrolytes, RSC Monographs, as cited previously herein.
Recently, a new strategy has been proposed by T. Yamamoto, M. Inami and T. Kanbara in Chem. Mater. 6 (1994) 44-50, and by X. Wei and D. F. Shriver in Chem. Mater. 10 (1998) 2307-2308, based on the use of relatively rigid polymer hosts with comparatively high values of glass transition temperature, Tg.
Yamamoto et al. describe the preparation and properties of polymer solid electrolytes based on poly(vinyl alcohol) (PVA) and poly[arylene(1,3-imidazolidene-2,4,5-trione-1,3-diyl)] (poly(parabamic acid), PPA).
Wei et al. describe two rigid polymer systems of poly(vinylene carbonate) (PVIC) (I) and poly(1,3-dioxolan-2-one-4,5-diyl oxalate) (PVICOX) (II) for the preparation of polymer electrolytes. The systems are said to display both favourable conductivity and mechanical properties.

The conductivities of the PVICOX system (II) are shown to be about 2-4 orders of magnitude higher than those of the PVIC system (I).
Wei et al. hypothesise that this is due to the irregularity of the PVICOX system (II) which frustrates close packing, thereby increasing static free volume and conductivity.
The developments of Yamamoto et al. and Wei et al. seem to go against the accepted precepts of polymer electrolyte operation. However, surprisingly high conductivities have been reported.
Although many other polymer architectures have been assessed as host polymers, prior to the publication of the results reported by Yamamoto et al., the ethylene oxide unit was widely believed to be the best option for high ionic conductivity as a consequence of its unique combination of structural and thermodynamic factors.
The results obtained by Yamamoto et al. and Wei et al. confirm that even polymers which do not contain the ethylene oxide structural unit may be effective media for the dissolution and efficient transport of charged species.
It has been shown in the art that the introduction of small polar molecules into the polymer network often results in a marked improvement in the observed ionic conductivity of the polymer.
Cameron et al., as previously cited herein, noted a decrease in viscosity and an increase in conductivity when 10 wt. % of the plasticisers tetrahydrofuran (THF) or propylene carbonate (PC) were added to poly(tetrahydrofuran) (PTHF) and to a copolymer of ethylene oxide and propylene oxide (50:50 by weight).
R. Huq, R. Koksbang, P. E. Tonder and G. C. Farrington reported the results of a study on the effect of a mixed plasticiser on the conductivities and the physical/chemical properties of radiation-polymerised polyether electrolytes. The mixed plasticiser compositions of ethylene carbonate and propylene carbonate containing 1M lithium hexafluoroarsenate (V) (LiAsF6) were found to possess better thermal, mechanical and lithium cycling properties.
M. S. Michael, M. M. E. Jacob, S. R. S. Probaharan and S. Radhakrishna reported in Solid State Ionics 98 (1997) 167-174 that a novel class of esters of benzene-1,2-dicarboxylic acids such as dioctyl phthalate (DOP), dibutyl phthalate (DBP) and dimethyl phthalate (DMP) had been used as plasticisers in high molecular weight PEO-lithium perchlorate matrix to improve the room temperature ionic conductivity of polymer-salt complex.
Although this improvement in conductivity in certain electrolyte systems has been interpreted in terms of an alteration in the transport mechanism (F. Croce, S. D. Brown, S. G. Greenbaum, S. M. Slane and M Salomon, Chem. Mater. 5 (1993) 1268-1272) or plasticisation of the polymer structure (Cameron et al. as cited previously herein), other effects may also contribute.
There has now been found a novel class of polymer electrolytes based on polymers that possess good ionic conductivity and excellent mechanical properties.